In any digital transmission system, it is necessary to recover the timing of the information being sent. Often it is considered inefficient to sent a separate clock signal, and so it is necessary to recover the timing from the data signal itself. In optical transmission systems this is usually done after conversion into electrical form, using one of two methods.
Firstly, a phase locked loop can be used to lock a separately generated clock to transitions in the data signal. Short periods where there are no transitions in the incoming data signal are bridged by the clock generator, which needs to be sufficiently stable.
Secondly, the incoming data can be filtered to extract a clock signal. In this case a resonant filter will be necessary, to fill in periods where there are no transitions. Furthermore, if the data is coded in non return to zero (NRZ) code for example, there may be substantially zero energy at the clock frequency. In these cases, a non-linear operation such as a squaring function is necessary to create a response, preferably a peak, at the clock frequency. A narrow band resonant filter can then be used to extract the clock frequency.
In optical transmission systems, such clock recovery arrangements have been used in regenerators and in receivers. However, electronic devices become expensive at high data rates, particularly when there is no other requirement for optical to electrical conversion.
There have been many attempts to implement more and more regenerator or receiver circuitry in optical form for improved performance or reduced cost. It is known to achieve a non linear function using an optical to electrical conversion device, then extract the clock frequency with a filter.
An early all optical regenerator is known from U.S. Pat. No. 5,446,573 using a non linear ring resonator comprising a semi conductor laser and phase modulators. However, it is difficult to make a practical device or integrate the arrangement.
Partly optical regenerators are known, where latching of the optical data input is carried out optically, following conventional electrical clock recovery. Optical clock recovery circuits were restricted to the use of mode-locked laser arrangements, eg as shown in U.S. Pat. No. 5,548,433. As shown in FIG. 1, a coupler 1 is used to couple an input optical data signal to a laser 2. The phase and frequency of an output pulse stream is locked to the input, since the laser acts as a resonant narrow band filter. However, such methods are limited to use with optical data signals such as return to zero (RZ) coded signals which have sufficient energy at the clock frequency, unlike NRZ coded signals. Most high capacity optical transmission systems use NRZ coding.
Another arrangement which is limited to use with RZ coded signals is known from U.S. Pat. No. 5,574,588. An optical phase locked loop is created by detecting correlation between the input signal and a new clock signal by combining them and passing them through an optical amplifier. U.S. Pat. No. 5,504,610 shows achieving such an optical locked loop using an optical mixer to multiply two inputs. This correlation process requires that substantial energy be present in the data at the clock frequency.
One document which tries to address the limitation to RZ coded signals is U.S. Pat. No. 5,434,692, which discloses a device for use with NRZ, CMI (Code, Mark Inversion) and biphase coded data. First, a passive filter delay interferometer is used to linearly add the input data to itself. The fibre 10 needs to be lengthy in high bit rate systems, probably many kilometers, to avoid beat noise. A three level signal is produced, which drives an optical amplifier 11 for am to pm conversion.
This, together with a narrow band filter 12, give a non linear function which produces a response at the clock frequency. A resonant filter 13 can then extract the clock.
Such a device is not practicable for commercial data transmission systems because it is bit rate specific, impossible to integrate, and difficult to tune. The narrow band filtering element in particular would require piezo electric devices for tuning, which are insufficiently reliable for field use. Furthermore, for each different type of coding, the XOR logical operation would need to be changed.